A display device using a liquid crystal compound has been widely used for a display for a watch, a calculator, a personal computer and so forth. The display devices utilize a refractive index anisotropy, a dielectric anisotropy and so forth of the liquid crystal compound.
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes a phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and polymer sustained alignment (PSA) mode. A classification based on a driving mode of a device includes passive matrix (PM) and active matrix (AM). The passive matrix (PM) is classified into static, multiplex and so forth, and the AM is classified into thin film transistor (TFT), metal insulator metal (MIM) and so forth.
The liquid crystal display device contains a liquid crystal composition having suitable physical properties. In order to improve characteristics of the device, the composition preferably has the suitable physical properties. General physical properties necessary for the liquid crystal compound being a component of the composition are as follows:
(1) being chemically stable and physically stable;
(2) having a high clearing point (clearing point: a phase transition temperature between a liquid crystal phase and an isotropic phase);
(3) having a low minimum temperature of liquid crystal phases (a nematic phase, a smectic phase or the like), in particular, a low minimum temperature of the nematic phase;
(4) having an excellent compatibility with other compounds;
(5) having a large dielectric anisotropy;
(6) having a large refractive index anisotropy; and
(7) having a small viscosity
When a composition containing a liquid crystal compound being chemically and physically stable as described in property (1) is used for the display device, a voltage holding ratio can be increased.
Because a composition containing a compound having a high clearing point or a low minimum temperature of liquid crystal phases as described in property (2) or (3) has a wide temperature range of the nematic phase, the device can be used in a wide temperature range.
In order to develop characteristics that are difficult to be output by a single compound, the liquid crystal compound is generally used in the form of the liquid crystal composition prepared by mixing with a number of other liquid crystal compounds. Accordingly, the liquid crystal compound to be used for the device preferably has a good compatibility with other compounds as described in property (4).
The liquid crystal display device having an excellent display performance in contrast, display capacity, response time characteristics and so forth has been required in recent years. For example, in order to decrease a driving voltage of the device, a liquid crystal compound allowing a decrease of a threshold voltage of the composition is required.
As is well known, the threshold voltage (Vth) is represented by the following equation (see H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27, 81 (1975)):Vth=π(K/∈0Δ∈)1/2 wherein, in the formula, K is an elastic constant and ∈0 is a dielectric constant of vacuum. As is understood from the equation, in order to decrease the threshold voltage (Vth), two ways are conceivable, in which the dielectric anisotropy (Δ∈) is increased or the elastic constant (K) is decreased. However, the elastic constant is not easily controlled with the present art. Therefore, a compound having a large dielectric anisotropy is ordinarily used to respond to a demand. Under such circumstances, the liquid crystal compound having the large dielectric anisotropy as described in property (5) has been actively developed.
Furthermore, in order to obtain a good display performance, a thickness of a cell of the liquid crystal display device for constituting the display performance, and a value of refractive index anisotropy (Δn) of the composition to be used are preferably constant (see E. Jakeman et al., Phys. Lett., 39A., 69 (1972)). Moreover, a response speed of the device is inversely proportional to a square of the thickness of the cell to be used. Therefore, in order to manufacture a device that can be applied to displaying moving images and so forth and also can respond at a high speed, a composition having a large refractive index anisotropy should be used. Accordingly, the liquid crystal compound having the large refractive index anisotropy as described in property (6) has been required.
Moreover, the response speed of the device is correlated with viscosity of the liquid crystal composition. Therefore, in order to manufacture the device that can respond at a high speed, a composition having a small viscosity should be used. Accordingly, the liquid crystal compound having the small viscosity as described in property (7) has been required.
As the liquid crystal compound having the large dielectric anisotropy, the large refractive index anisotropy and the small viscosity, a variety of difluorostilbene derivatives having a positive dielectric anisotropy have been prepared so far. For example, Patent literatures Nos. 1 to 6 show two-ring compounds. However, the compounds have an insufficiently high clearing point. Thus, when the compounds are combined into the composition, a temperature range in which the composition is used as the device is insufficiently wide.
Moreover, Patent literatures Nos. 3 to 6 show three-ring compounds, and Patent literature No. 4 shows compound (S-1), compound (S-2) and so forth. However, the compounds have an insufficiently large dielectric anisotropy, and furthermore compatibility with other compounds are insufficient in many cases.
